Flavonoids comprise a diverse collection of red to blue colored secondary metabolites that accumulate in the tissues of many plant species. The primary structure of flavonoids consists of two aromatic carbon groups; benzopyran (A and C rings) and benzene (B ring). The variation in the heterocyclic C-ring of flavonoids and the interlinkage between the benzopyran and benzene groups are the basis for the classification of flavonoids into the flavone, flavonol, flavonone, isoflavone, anthocyanin, and flavane groups.
Anthocyanins have been associated with many important physiological and developmental functions in the plants, including, modification of the quantity and quality of captured light (Barker D H et al,. Plant Cell and Environment 20: 617-624, 1977.); protection from the effects of UV-B radiation (Burger J and Edwards G E. Plant and Cell Physiology 37: 395-399, 1996; Klaper R et al., Photochemistry and Photobiology 63: 811-813, 1996); defense against herbivores (Coley and Kusar. In: Mulkey S S, Chazdon R L, Smith A P, eds. Tropical Forest Plant Ecophysiology. New York: Chapman and Hall 305-335, 1996); and protection from photoinhibition (Gould K S, et al., Nature 378: 241-242, 1995; and Dodd I C et al,. Journal of Experimental Botany 49: 1437-1445, 1998); and scavenging of reactive oxygen intermediates in stressful environments (Furuta S et al., Sweetpotato Res Front (KNAES, Japan) 1:3, 1995; Sherwin H W and Farrant J M., Plant Growth Regulation 24: 203-210, 1998; and Yamasaki H Trends in Plant Science 2: 7-8, 1997).
Anthocyanins have demonstrated anti-oxidant activity, suggesting a role in protecting against cancer, cardiovascular and liver diseases (Kamei H et al., J Clin Exp Med 164: 829, 1993; Suda I, et al., 1997. Sweetpotato Res Front (KNAES, Japan) 4:3, 1997; and Wang C J, et al., H Food Chem Toxicology 38: 411-416, 2000). Thus, anthocyanin-rich foods and extracts have been studied for their utility in a variety of therapeutic applications (e.g. Katsube et al., J Agric Food Chem (2003) 51(1):68-75; Renaud et al., Lancet (1992) 339:1523-1526; and Natella et al., J Agric Food Chem (2002) 50(26):7720-7725). There is also interest in the use of anthocyanin-rich plant species in the production of natural dyes (Venturi and Piccaglia, “The Rediscovery of Dye Plants as Promising “Non Food Crops””, Interactive European Network for Industrial Crops and their Applications, Newsletter no. 10, November 1999).
Many steps in anthocyanin biosynthesis are shared among plant species, while the regulatory elements that underlie the expression level and pattern of genes encoding these enzymes are diverse. In Petunia, AN2 encodes a MYB domain protein that is orthologous to C1 from maize (Quattrocchio F et al., 1999, Plant Cell 11: 1433-1444), and Arabidopsis genes PAP1 and PAP2 (Borevitz et al., Plant Cell. 2000 December;12(12):2383-2394). The Anthocyanin1 gene (AN1) of petunia encodes a basic helix-loop-helix (bHLH) protein that activates the transcription of the structural anthocyanin gene Dihdroflavonol Reductates (DFR). The expression of AN1 is regulated by AN2 (Spelt et al., Plant Cell. 2000 September;12(9):1619-32). In Arabidopsis, two other transcription factors have been implicated in controlling the accumulation of flavonoids: the homeodomain protein Anthocyaninless2 (ANL2) is required for anthocyanin accumulation in subepidermal cells, while and the zinc finger protein, TT1, is involved in the accumulation of proanthocyanidin polymers in the seed coat (Kubo et al., Plant Cell. 1999 July;11(7):1217-26; Sagasser et al., Genes Dev. Jan. 1, 2002;16(1):138-49).
Isoflavones have also been widely studied for their potential therapeutic utility and health benefits (Hewitt and Singletary, Cancer Lett (2003) 192(2):133-143; Katz, J Altem Complement Med (2002) 8(6):813-821). Isoflavones play roles in plant pathogen response and in symbioses with rhizobial bacteria (Pueppke et al. 1998, Plant Physiol 117:599-608). They occur almost exclusively in soybeans and other legumes (Jung et al. 2000, Nature Biotechnology 18:208-212). Three principle isoflavone aglycones occur in soybean: daidzein, genistein and glycitein. Glycitein accounts for only about 10% of the total isoflavone content (Song et al 1999, J Agric Food Chem 47:1607-1610), but some research suggests glycitein is both more bioavailable (Song et al 1999, J of Nutr. 129:957-962) and more estrogenic (Songe et al 1999, J Agric Food Chem, supra) than daidzein and genistein.